1. Field of the Invention
The present invention relates to the catalytic reduction of halonitroaromatic compounds into haloaminoaromatic compounds via hydrogenation in the presence of catalysts based on modified Raney nickel.
The present invention especially relates to such modified Raney-type catalysts, per se.
2. Description of the Prior Art
Catalysts of Raney nickel are widely used in hydrogenation reactions. They are prepared by alkaline attack, using a concentrated base, on aluminum-rich aluminum/nickel alloys. This attack has the essential consequence of removing aluminum. The catalysts obtained are nickel crystallite agglomerates having a high specific surface and a variable residual aluminum content. In this respect, see the following literature references:
(a) M. Khaidar, C. Allibert, J. Driole and P. Germi, Mat. Res. Bull., 17, 329-337 (1982); PA1 (b) V. Birkenstock, R. Holm, B. Reinfandt and S. Storp, J. Catal., 93, 55-67 (1985); PA1 (c) J. Gros, S. Hamar-Thibault and J. C. Joud, Surface and Interface Analysis, 11, 611-616 (1988). PA1 (a) 50.ltoreq.Ni.ltoreq.78, preferably 60.ltoreq.Ni.ltoreq.70, PA1 (b) 20.ltoreq.Al.ltoreq.40, preferably 25.ltoreq.Al.ltoreq.35, and PA1 (c) 2.ltoreq.Mo.ltoreq.20, preferably 5.ltoreq.Mo.ltoreq.15. PA1 (i) under a H.sub.2 pressure, P.sub.H.sbsb.2, ranging from 1 to 100, preferably from 5 to 50, and even more preferably from 15 to 25 bar (1 bar=10.sup.5 Pa), PA1 (ii) the reaction temperature ranging from 25.degree. to 150.degree., preferably from 40.degree. to 120.degree., more preferably from 55.degree. to 80.degree. C.
The reduction of halonitroaromatic compounds, such as 3-chloro-4-fluoronitrobenzene, into haloaminoaromatic compounds, such as 3-chloro-4-fluoroaniline, is of great importance in organic chemistry because it provides a key route in the production of pharmaceutically active principles.
One of the disadvantages in this reduction via hydrogenation in the presence of Raney nickel or similar compounds is a hydrodehalogenation side reaction downstream of the reduction of the nitro functional group. This reaction is carried out only by means of the halogenated aniline. Such hydrodehalogenation of the aniline nucleus results, in addition to its negative influence on the selectivity and the yield of the catalytic hydrogenation reaction, in the formation of particularly aggressive byproducts, especially vis-a-vis catalysts of Raney nickel type. These byproducts include, for example, hydrochloric acid, which has the effect of significantly decreasing the useful life of the catalyst. Hydrodehalogenation is particularly critical in the event of low substrate concentrations.
International Patent Application PCT 89/07,096 describes exactly this problem of catalytic hydrogenation of halonitroaromatic compounds. The process described therein features the use of a Raney catalyst based on cobalt/aluminum/nickel/chromium. Even though this process provides improvements with respect to catalysts of activated nickel or activated carbon/platinum type, optionally in combination with catalyst inhibitors, or also with respect to other catalysts of sulfur-modified platinum type, the hydrodehalogenation side reaction persists and continues to produce not insignificant amounts of undesirable halogenated byproducts and dehalogenated compounds.
Canadian Patent CA-961,834 describes Ni(85% to 96%)/Mo(0.5% to 10%)/Al (.ltoreq.14%) catalysts which can be used for catalytic hydrogenation, especially of carbonyl compounds such as, for example, acetone, nitrophenoxide, itaconate, and the like.
The substrates involved are different from those associated with the catalysts and the hydrogenation process in accordance with this invention. These catalysts are neither selective nor significantly active with respect to nitro radicals borne by aromatic nuclei. In addition, this patent is conspicuously silent as regards hydrodehalogenation.
Similarly, the article by S. Hamar-Thibault et al, J. Chim. Phys., 88, 219-232 (1991), describes Raney catalysts doped with molybdenum and employed in the hydrogenation of acetophenone in the liquid phase. Here again, the problem of hydrodehalogenation is not encountered. In addition, this article also describes that the more the molybdenum content of the catalyst increases, the more the intrinsic activity and the specificity with respect to sites to be hydrogenated, i.e., carbonyls, decrease.
Among other catalytic hydrogenation starting materials which are not affected by the hydrodehalogenation side reaction, exemplary are the dinitroaromatic compounds reduced to diaminoaromatic compounds via the process described in DE-3,537,247. This particular hydrogenation process employs catalysts based on Raney nickel doped with molybdenum. These finished catalysts are depleted in aluminum, relative to the amount of Mo present, as is illustrated by the Al/Mo ratio, which is less than 1 (Al and Mo being expressed as % by weight) for the Raney catalysts described.
Finally, Raney catalysts are also known which are doped with a transition metal element. These Ni/Al/metal monodoped hydrogenation catalysts are used for the selective reduction of para-chloronitrobenzene to para-chloroaniline. The presence of a metal dopant in the Raney catalyst promotes a decrease in selectivity and a decrease in catalytic activity. Moreover, the dopant appears to increase hydrodehalogenation. The latter even remains significant for high paranitrochlorobenzene concentrations (case of iron and chromium). Thus, in a haloaromatic environment, doping Raney catalysts with a metal does not appear desirable.